The technical domain in question is more especially that of pipes made of concrete, reinforced or not, intended for making underground flow networks for the circulation of domestic sewage, industrial waste water or rain water.
A major imperative which must be respected when constructing such underground networks resides in the tightness between the ends of the interfitted pipes.
In fact, the joins of the pipes must be perfectly water-tight in order to avoid either pollution of the surrounding ground, in the base of circulation of highly polluted water, or the introduction of water from the ambient medium, of which the additional proportion would be such as to disturb operation of sewage purification or processing stations initially designed to absorb a given volume per unit of time.
To this end, it has therefore been proposed to provide each end of the pipes with a female end in the form of a neck and a male end whose outer diameter is smaller than the inner diameter of the neck end. The inner and outer peripheral surfaces of these ends are shaped so as to allow the intermediate assembly of a gasket generally designed to perform two inherent functions.
The first is to centre the two ends after introduction and the second function is to provide peripheral tightness after coupling.
A third function, generally following therefrom, is that of facilitating penetration of the male end in the neck end, so as to allow appropriate coupling up to the position of maximum introduction defining a state of tight centering of the male end inside the neck end.
It was initially proposed to use a gasket in the form of a toric element, initially placed in a groove defined in the male end or the female end.
It will be appreciated that such a gasket does not offer a capacity of elastic deformation which is sufficient to comply with the first imperative of centering. Consequently, if such a gasket is used, the cylindrical bearing surfaces having to face each other must necessarily be made with precision.
In the particular case of concrete, such a requirement becomes prohibitive due to the technical means that must be employed.
To overcome this drawback, it has been proposed to use gaskets comprising a portion designed to provide relative facility of introduction of the ends and to provide an appropriate tightness, and a local portion adapted to withstand the radial stresses transmitted by the surrounding medium, in order to maintain a centering between the pipes.
The personnel assembling the pipes on site must take particular care with such gaskets. In fact, the pipe ends must be maintained perfectly clean and each gasket must be placed correctly, in the selected direction, so as to allow coupling under suitable conditions and to obtain a water-tight centering of the assembled pipes.
It has been observed that this operation is generally delicate to carry out, in view of the mass of the pipes having to be relatively displaced, and involves particularly well-qualified manpower which is not always available on site.
Furthermore, a fairly large number of gaskets of different shapes are available on the market, and the choice or use of these gaskets must therefore take into account their natural compatibility with the pipes made.
In an attempt to overcome these various drawbacks, certain manufacturers have envisaged immediately incorporating a gasket in the female or neck end, so as to avoid random positioning and to facilitate the personnel's work on site.
To this end, certain propositions envisaged defining a housing in the inner peripheral wall of the neck end, in order to maintain a gasket of appropriate complementary shape therein, under stress. In practice, it has been ascertained that this technique presented three major drawbacks.
The first is that the manufacturer is obliged to purchase gaskets, even for pipes made in an advance manufacturing programme and intended to be stored in a storage yard for a period which is sometimes indefinite. The manufacturer is heavily penalized by this obligation.
The second resides in the fact that a gasket, even placed under stress inside a housing of a female end, undergoes in time an appreciable relaxation of the stress imposed thereon. It is frequently observed that such gaskets no longer properly occupy the housing reserved therefor after a relatively long storage time or even separate therefrom entirely, thus posing a problem of replacement for the assembly teams on site.
The third drawback resides in the necessity to have available specific equipment for making such housings provided in advance. In particular, it is necessary to allow for considerable investment for the purchase of moulding rings having to be positioned on the forms for casting and setting each pipe.
As in certain cases the use of pipes of ordinary shapes must always be envisaged, this obligation imposes two moulding installations on the manufacturer, this considerably increasing the operational costs of a unit for manufacturing such pipes.
Furthermore, another proposition is known, consisting in providing a gasket with an undeformable rigid core, in the form of a ferrule, embedded in a mass of elastically deformable material forming annular mouldings on the outer and inner peripheral faces.
These gaskets undoubtedly provide advantages over the heretofore known solutions, as the rigidity of the core allows positioning in a neck end without risk of untimely displacement.
However, these gaskets present a drawback in that they must be made by moulding, in order to obtain a final product of monolithic structure with a resistant bond between the rigid core and the deformable material.
The obligation of making such gaskets by moulding does not enable particular sections in transverse cross-section to be given to the elastically deformable mouldings or, in other cases, does not enable the undeformable core to be completely coated.
In any case, the gasket thus designed can therefore not be exactly adapted to needs and, in particular, cannot present the faculty of compensation in relation with the manufacturing tolerances which are different between a male end and a female end, taking into account the process for manufacturing a pipe.